Mock IOptionsMonitor

You are calling the constructor of the OptionsMonitor<TOptions> class incorrectly.

In this case I would have just mocked the IOptionsMonitor<AuthenticationSettings> interface

For example using Moq

AuthenticationSettings au = new AuthenticationSettings() { ... };
var monitor = Mock.Of<IOptionsMonitor<AuthenticationSettings>>(_ => _.CurrentValue == au);
ActiveDirectoryLogic _SUT = new ActiveDirectoryLogic(monitor);